1. Field of the Invention
The present invention relates generally to electrical circuits, and more particularly but not exclusively to circuit elements for adjusting the oscillation frequency of a controlled oscillator.
2. Description of the Background Art
In a digitally controlled oscillator, the operating frequency is adjusted by enabling or disabling a number of tuning circuit elements. The tuning circuit elements can be any components and circuits, such as transistor or varactor. A tuning circuit element is enabled and disabled by setting its binary control signal to a binary one and a binary zero, respectively. Because of the binary nature of the control signal, the oscillation frequency of the digitally controlled oscillator is quite insensitive to the variations of power supply voltages. On the other hand, the oscillation frequency of the digitally controlled oscillator is still sensitive to the ambient temperature variations. The temperature influence on the oscillation frequency of the digitally controlled oscillator is of interest. In a typical digitally controlled oscillator, the oscillation frequency can differ by more than 1% over 100° C. temperature variations.
In order to provide a stable reference clock over voltage variations, a digitally controlled oscillator is employed. A method and an apparatus are provided in the present invention to adjust the silicon temperature close to a predetermined value such that the oscillation frequency of the digitally controlled oscillator is less sensitive to the changes of the ambient temperature in any case.